The invention relates generally to electronic packaging. In particular, the invention relates to electronic packaging for sensor arrays and a method of making the same.
As integrated circuit device densities increase and device sizes shrink, system performance is increasingly impacted by limitations in interconnect technology and packaging of the chips used to fabricate the devices. For example, package limitations, such as the maximum allowable number of chip input/output contacts, have resulted in the inability to utilize all of the chip's capabilities. Multi-chip packaging generally requires wide spacing of chips to accommodate wiring channels, which results in longer chip interconnects, increased parasitic capacitance and a decrease in system speed. Moreover, complex packaging structures may be expensive and unreliable.
Current x-ray detectors generally rely on standard electronic packaging and interconnect technology, processes, materials etc., including printed circuit boards, surface mount technology (SMT) packages, ceramic substrates, etc. However, current interconnect and packaging technologies, which utilize rigid substrates for fabrication are limited in resolution and pitch capabilities. Accordingly, high density packaging of the order of 100 to 800 contacts/cm2 are desirable to achieve the full potential of x-ray detectors. Disadvantageously, such high densities may not be possible to achieve with rigid substrates for chip interconnect.
In the case of photon counting X-ray detectors, it is generally desirable to have pixel pitch smaller than 1 mm. This enables the detector to be used at higher X-ray flux. Typically, these detectors are limited to counting photons at a rate of 1 million counts per second. For pixels with an area of approximately 1×1 mm2, the corresponding maximum flux rate is 1 million counts per second per mm2, whereas for pixels with an area of 0.5×0.5 mm2, the maximum flux rate is 4 million counts per second per mm2. Even finer pixel pitch would allow higher flux rates to be counted, thereby benefiting applications where high statistical significance is needed. For example, in X-ray imaging applications, the contrast to noise ratio is a quality metric which is proportional to the square root of the number of x-rays counted. Many benefits of photon counting detectors are dependent on the ability to count sufficient number of x-rays in order to achieve statistical significance. Therefore it is advantageous to provide a packaging technology to achieve high interconnect density, where small area sensor pixels are routed to readout electronic inputs.
Sometimes, multilayer ceramic substrates (MLCs) are used as alternatives to standard electronic packaging. Today's chips require more connections between the chip and the MLC and also require connections on denser grids. This requirement makes it difficult to make MLCs economically, exacerbates electrical noise problems and causes yield problems since redistribution planes of greater complexity will be needed.
Flexible substrates may be used to achieve relatively high density input/output contacts. However, during fabrication, these flexible substrates (e.g., polyimide) are subjected to temperatures and mechanical forces that cause the substrate material to stretch, shrink and otherwise change in physical dimensions both during and upon completion of processing. These dimensional changes and instability can be minimized, but not to the extent that it is currently feasible to produce ultra fine pitch multi-layer flexible interconnect with trace pitches at or under 0.030 mm, at high yield. Due to these limitations flexible substrates are costly to fabricate and employ as the complete interconnect substrate.
Therefore, there is a need to obtain a simpler method of making high density electronic packaging and low cost interconnect components. There is also a need for allowance for testability of components before connecting to the assembly.